The invention generally relates to optical instruments and methods. More particularly, the invention relates to a confocal laser system for scanning a surface or other object with a laser beam and generating an image of the object.
Optical scanning imaging techniques are employed in devices such as scanning laser microscopes (SLM), confocal scanning laser microscopes (CSLM), tandem scanning confocal microscopes (TSM), scanning laser ophthalmoscopes (SLO), and flying spot television (FSTV) devices. Confocal imaging systems can provide enhancements in contrast and in dynamic range. Certain of these imaging systems include moving optical elements for deflecting a laser beam, so that an illumination spot is swept across the object to be scanned. Other such systems employ mechanical elements to rotate an illuminated pinhole for the same purpose. In the TSM, a plurality of illumination spots is moved simultaneously, to provide source multiplexing, necessary because the source does not have the higher radiance (brightness) of a laser.
A double scanning optical apparatus is disclosed in U.S. Pat. No. 4,764,005 of Webb et al., the teachings of which are incorporated herein by reference. The apparatus utilizes multiple scanning elements, including a multifaceted rotating polygonal reflector scanner, to provide scanning of both incident and reflected light at television-rate frequencies.
Additionally, certain flying spot imagers us a cathode ray tube (CRT) as a light source, with a single illuminated point scanned across the CRT face. The tube face is imaged onto the object to provide the illumination raster.
A TSM is discussed in Petran et al., "Tandem-Scanning Reflected-Light Microscope," Journal of the Optical Society of America, Vol. 58, No. 5, pp. 661-664, May 1968. Petran et al. observe that reflected-light microscopy of semi-transparent material is usually unsatisfactory because of low contrast and light scattering. In the TSM, in which both the object plane and the image plane are scanned in tandem, only light reflected from the object plane is included in the image. In the Petran et al. system, the object is illuminated with light passing through holes in one sector or side of a rotating scanning disk, known as a Nipkow disc. The scanning disk is imaged by the objective at the object plane. Reflected-light images of these spots are directed to the diameterically opposite side of the same disk. Light can pass from the source to the object plane, and from the object plane to the image plane, only through optically congruent holes on diametrically opposite sides of the rotating disk. This configuration produces an image having enhanced contrast and sharpness relative to a conventional reflected-light microscope.
Tandem scanning confocal arrangements and flying spot CRT configurations, however, are "light-starved" by the limited brightness of the illumination spot. In TSM configurations, this brightness limitation is partially compensated by the multiplex operation. TSM systems, however, are hampered by stray light scattered from the moving pinhole array. A general reference for microscopy is The Handbook of Biological Confocal Microscopy, Pawley, 2nd. ed., Plenum, 1991.
A further advance in confocal scanning laser microscopy is disclosed in U.S. Pat. No. 5,028,802 (Webb et al. ) in which a laser source comprising a microlaser array scans the surface of an object. A beam splitter directs the light reflected from the object to a detector array. The detector array can be scanned in synchrony with scanning the microlaser array to detect light reflected from the object due to each microlaser in the laser source. Here, the "scanning" may be entirely an electronic switching operation, reducing the alignment, spatial filtering, optical aberration and mechanical distortion problems associated with most optical scanning arrangements.
An alternative approach to scanning laser microscopy is disclosed in Juskaitis et al., "Fibre-Optic Based Confocal Scanning Microscopy with Semiconductor Laser Excitation and Detection," Electronics Letters, Vol. 28, No. 11, pp. 986-988, May 1992; Juskaitis et al., "Fibre-Optic Based Confocal Microscopy Using Laser Detection," Optics Communications, Vol. 99, No. 12, pp. 105-113, December 1992; and Juskaitis et al., "Spatial Filtering by Laser Detection In Confocal Microscopy." Optics Letters, Vol. 18, No. 14, pp. 1135-1137, July 1993. As with previous systems, Juskaitis et al. image an object by projecting a laser beam at the object. However, instead of detecting reflected light from the object, Juskaitis et al. feedback the reflected light to the laser source. The laser source in turn, increases or decreases its power in response to the remitted light. Juskaitis et al. detect an image signal as a modulation on a power-monitor signal derived from a power monitor diode, located behind the laser source. Alternatively, in the case of a semiconductor laser, they measure the drive voltage to the laser.
Further improvements over the above discussed systems are desirable. By way of example, the aforesaid Webb et al. patent solves light starvation problems and uses no moving parts, however, requires an array of detectors that is scanned in synchrony with a microlaser source array to achieve confocality. Arrays of detectors can be costly and can complicate the design of the microscope. By way of further example, Juskaitis et al. attempt an alternative approach to using the reflected light from the object to develop the imaging signal. However, Juskaitis et al. employ mirrors to scan the image of a laser source over the object being scanned.
Accordingly, an object of the present invention is to provide an improved confocal laser imaging system that requires no moving parts.
Another object of the present invention is to provide a compact and reliable laser imaging system that generates an image by detecting output intensity of a laser source.
Other general and specific objects of the invention will in part be obvious and will in part appear hereinafter.